1. Field of the Invention
The present invention generally relates to a reflective liquid crystal cell driven by active matrices and more particularly relates to a reflective liquid crystal display driven by active matrices fabricated on either glass plates, Si-wafers or polymeric substrates.
2. Description of the Related Art
Conventional systems utilize transmissive and reflective active-matrix-driven liquid crystal displays (AMLCDs). The basic structure of AMLCDs is shown in FIG. 1 and generally includes polarizers 101 and 102 glass substrates 103 and 104, transparent conductive electrodes 105 and 106, color filters 108 (optional), on/off transistor switches 109, and an LC medium 110 sandwiched between two transparent conductive electrodes 105 and 106. A back-light source 107 illuminates the display panel from below. Alignment layers (not shown) such as rubbed polyimide films are typically disposed between the LC medium 110 and the transparent conductive electrodes 105 and 106.
For reflective AMLCDs with reflective electrodes built inside the LC cell, the transparent conductive electrode 106 is usually replaced by a reflective metal electrode which occupies a larger area to cover the transistor 109. Also for reflective AMLCDs, there is no need for the back-light source 107. Instead, ambient light or another light source illuminates the display panel from the top of FIG. 1. The transmissive-type AMLCD typically includes repetitive unit cells or picture elements (pels). FIG. 1 illustrates a 3 by 3 matrix of pels.
A schematic drawing of a pel is shown in FIG. 2, where the attached numbers have correspondingly the same meanings as in FIG. 1. The capacitor 111 denotes the capacitance of the LC medium 110 sandwiched between two transparent conductive electrodes 10S and 106, and capacitor 120 denotes the storage capacitor which provides a parallel capacitance to the LC capacitance 111 and is terminated on a line 121 common to all the storage capacitors in the display. An alternative design shown in FIG. 2 includes a storage capacitor 122 between the electrode 106 and the gate bus line 107.
When a voltage below a threshold voltage is applied to the gate line 107, the transistor 109 is in an off-condition so that the potential on the data bus line 108 and electrode 106 are isolated from one another. When a voltage larger than the threshold voltage is applied on the gate bus line 107, the transistor 109 is in an on-condition (low impedance state), thereby allowing the voltage on the data bus line 108 to charge the electrode 106. Varying the voltage to the electrode 106 controls the liquid crystal cell 111 such that different amounts of light are transmitted across the liquid crystal display, thus resulting in the display of a gray scale of light.
A reflective-type AMLCD is similar in structure to the transmissive-type AMLCD; however, the transparent electrode 106 is usually replaced with a reflective metal electrode which generally occupies a larger area to cover the transistor 109. Also for reflective-type AMLCDs, there is no need for the back-light source 107. Instead, ambient light or another light source illuminates the display panel from the top.
There are several materials such as indium oxide, tin oxide indium-tin oxide (ITO), zinc oxide, indium-zinc oxide (IZO) that can be used for the transparent electrodes of the transmissive-type and reflective-type AMLCDs. Indium-tin oxide is the preferred choice because it has good transparency in the visible light, suitable conductivity, and is inexpensive to manufacture. In the state-of-art transmissive-type AMLCD, both electrodes 105 and 106 are ITO, and rubbed polyimide (PI) films made of the same PI resin are on each ITO electrode to form LC alignment layers for the LC medium in the display.
In the polyimide-aligned liquid crystal display art, it is well-known that there is charge injection from the electrode into the adjacent aligning PI film such that the exact potential across the LC medium is determined by the applied voltage across the electrodes, the difference in surface potentials resulting from the charge injections into the aligning PI films from the adjacent electrodes, and the work-function difference between the opposite electrodes. In the case of transmissive-type AMLCD using the same ITO and PI on opposite sides of the LC medium, the differences in work function and surface potential are zero because of the symmetric arrangements of both the electrodes and alignment layers.
Therefore, the exact voltage drop across the LC medium is determined only by the applied voltage across the two display electrodes for the transmissive-type AMLCD, and is approximately equal to that of the applied voltage if the thickness of the aligning PI film is negligible compared to the thickness of the LC medium.
For the reflective-type AMLCD, there is usually a difference in work function between the transparent electrode, such as ITO with a work function about 4.7 eV, and the reflective electrode, such as Al with a work function ranging from 4.06 to 4.41 eV depending on the process conditions. Furthermore, it is very difficult to balance out this difference in work function across the whole display panel at a given time resulting in perceivable flicker on those unbalanced locations on the display.
In addition, the Al electrode is more reactive to the adjacent PI film than ITO electrode in terms of charge injection into the PI films, resulting in a substantial difference in surface potentials on the PI-to-LC interfaces situated at opposite sides of the LC medium of a reflective-type AMLCD. This net surface-potential difference is not uniform across the display panel, not stable under light illumination electrical driving, and temperature variation. As a result, there exists a time-varying DC field across the LC medium in the LC cell even if the voltage applied across the two electrodes of the LC display is AC, that is, lacking a DC component.
This DC field can be reduced to zero by applying a suitable DC voltage defined as “Vcom shift” on the ITO (or common) electrode of the LC display. The “Vcom shift” not only changes with time at each location, but also varies among different locations at the same time, resulting in a flickering display if it is driven by the frame-inversion method (e.g., with a frame rate lower than about 70 Hz). The larger the Vcom shift, the higher the frame rate required to avoid flickers.
Due to the spatial variation and temporal drift of this “Vcom shift”, a flickerless display can only be achieved by driving the display with column-inversion methods at a frame rate higher than about 70 Hz, resulting in lower brightness and larger power-consumption than if the display had been driven by the frame-inversion method. A stable and uniform “Vcom shift” across the whole display panel is a necessary condition to achieve flickerless display with lower power consumption and higher brightness.